The present invention relates to the rotation of thin disks. More specifically, the present invention relates to an improved roller for rotating semiconductor substrates within a tank of fluid.
A conventional method for cleaning particles from semiconductor wafers submerged within a tank of fluid is known as megasonic cleaning. During megasonic cleaning, a transducer oscillates between compressed and strained states at a rate near 1 MHz. The megasonic oscillation output by the transducer is coupled to the fluid contained within the tank, causing pressure oscillation therein. As the pressure in the fluid oscillates between positive and negative, cavitations or bubbles form in the liquid during negative pressure and collapse or shrink during positive pressure. This bubble oscillation gently cleans the surface of the wafer.
In practice, megasonic cleaners experience a number of limitations. For instance, the intensity of the cleaning energy experienced across the wafer""s surface often decreases with increased distance from the transducer. This energy gradient necessitates wafer rotation in order to achieve uniform cleaning across the entire wafer surface.
Conventionally, to achieve rotation, a wafer is positioned on a pair of rollers each of which are mounted to a shaft that extends through the wall of the megasonic tank. Outside the megasonic tank each shaft is coupled to a motor that when energized causes the shaft, and the roller attached thereto, to rotate. As the rollers rotate, so does the wafer positioned thereon.
Although such roller assemblies facilitate uniform wafer cleaning across the entire surface of the wafer, they also experience several drawbacks, such as causing cleaning fluid leakage. In an effort to prevent cleaning fluid leakage, rubber seals are used to seal between the roller shaft and the wall of the tank. In practice, however, the heat and chemistry of the tank fluid degrade the seals and leaking inevitably occurs. Fluid leaking undesirably increases the systems operating costs. To reduce this cost, the leaking fluid is sometimes collected, filtered and recirculated to the tank. The degrading seals and leaking chemistry present a potential particle source, and, because leaking rates are random, introduce variables to the cleaning process. Eventually the seals have to be replaced causing downtime costs, as well as labor and replacement parts costs.
Accordingly, a need exists for an improved method and apparatus for rotating a substrate as it is processed within a fluid tank.
The present invention overcomes the problems described above, by providing a roller which magnetically couples a driver for rotating the roller (i.e., an inventive roller/driver assembly). Because the roller and driver are magnetically coupled, no openings are required in the walls of the fluid filled processing tank, and no leakage occurs. In one aspect, an outwardly extending pocket is formed in or sealed to a wall of the processing tank, and a shaft of the inventive roller/driver assembly is supported by the pocket. This configuration has been found to provide excellent positional accuracy.
Because of the magnetic coupling between the driver and the roller, the roller may be easily installed and removed, often requiring no tools whatsoever. Further, in the pocket embodiment a magnet positioned on the roller is axially offset from a magnet positioned on the driver, the driver magnet being positioned outside (i.e., in a direction away from the tank) relative to the roller magnet. The axial offset causes the roller magnet to be attracted outwardly, away from the tank, and thus continuously pulls the roller shaft into the pocket. Accordingly, in certain embodiments, no rings or locking devices are required to lock either the roller assembly or the driver assembly in place, and no metal parts are exposed to the cleaning fluid chemistry, therefore corrosion may be avoided. Moreover, cleaning fluid leakage may be completely eliminated resulting in less wafer to wafer processing variations and less particle generation. Fluid filled processing tanks which employ the inventive roller/driver system may be able to provide higher throughput with lower consumable costs and higher, more consistent quality yields.
Other features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.